One-Session Treatment (OST) is a form of massed exposure therapy for the treatment of specific phobias. OST combines exposure, participant modeling, cognitive challenges, and reinforcement in a single session, maximized to three hours. Clients are gradually exposed to steps of their fear hierarchy using therapist-directed behavioral experiments. Although there are several studies in the literature examining the efficacy of OST, little has been done to summarize this research. In the following review, research on and empirical support for OST are reviewed with an emphasis on the types of stimuli, samples, and methodologies utilized. Research generally supports OST's efficacy, although replication by independent examiners using adult and child samples is needed using more rigorous comparisons (e.g., psychological placebo or other treatments). Overall, OST continues to be a promising treatment for specific phobias; however, a great deal more investigation is needed to identify mechanisms of change, mediators, and moderators. (Contains 4 tables.)